Manufacture of permanent magnets



Sept. 8, 1953 MANUFACTURE OF PERMANENT MAGNETS Filed July 10,- 1947 NEEL 2,651,105

3 Sheets-Sheet l fmmrz 2 OXALAj'E 1 J50 :7: m 4!; 4w mt fiwpspnruns or nzpucmuv INVENTOR LOUIS NEEL J7 45611 1, we?- Sept. 8, 1953 N v NEEL 2,

MANUFACTURE OF PERMANENT MAGNETS Filed July 10, 1947 s Sheets-Sheet s Patented Sept. 8, 1953 MANUFACTURE OF PERMANENT MAGNETS Louis Nel, Grenoble, France, assignor to Socit dElectro-Chimie, dElectro-Metallurgie et des Acieries Electriques dUgine, Paris,

corporation of France Application July 10, 1947, Serial No. 760,091 In France April '7, 1942 6 Claims. (Cl. 29-155.6)

The present invention relates to permanent magnets and their manufacture.

This application is a continuation-in-part of my copending application Serial No. 585,399, filed March 28, 1945, now abandoned.

I have discovered that permanent magnets having high coeflicients of quality (high coercive force combined with high remanence) may be made by the low temperature decomposition and reduction of certain carbon and oxygen containing compounds of ferromagnetic metal to produce a ferromagnetic metal powder containing a small amount of carbon and traces of oxygen, and shaping magnets from such powder by compacting it at a temperature below that at which objectionable crystal growth would occur. The compounds which I have found suitable are compounds of ferromagnetic metal containing carbon and oxygen which are decomposable and reducible at relatively low temperatures (below 500 C. and preferably between about 300 and 400 C.) to produce a ferromagnetic metal powder which when compacted into magnet form produces permanent magnets of high quality. The preferred compounds are formates, oxalates, carbonates and carbonyls of iron and other ferromagnetic metals and alloys. These compounds when decomposed are reduced at relatively low temperatures yield metallic powders in which the elementary grains or crystals are of colloidal dimensions of the order of .01 to 0.1 micron. Since such compounds contain both carbon and oxygen, carbon and oxygen are present during decomposition and reduction. Reduction is carried out so that the reduced metal powder contains a small but substantial amount of carbon (usually of the order of 0.2 to 2.5%) together with traces of oxygen (usually of the order of 1%). I believe that the carbon and the oxygen are present largely at the surfaces of the elementary grains or crystals and inhibit their growth not only during reduction but also during the compacting operation which is required for shaping the powder into magnets which are structurally strong and coherent. I believe that the inhibiting efiect of the carbon and perhaps of the oxygen is largely responsible for the high coefiicients of quality of my permanent magnets.

In the accompanying drawings:

Fig. 1 is a diagram showing the hysteresis loop of a permanently magnetic material upon which I have indicated how the so-called coefficient of quality of such material is determined;

Fig. 2 consists of curves showing the relation between the temperature of reduction of iron formate and iron oxalate and the coefficients of quality of magnets compacted from such products of reduction;

Fig. 3 is a diagram showing the variations of the coercive force and the remanence with the temperature of reduction of a magnet compacted France, a

. 2 from the powder reduction product of iron formate;

Fig. 4 is the upper left quadrant of a hysteresis loop diagram for magnets compacted from powders corresponding to points I, 2 and 3 of Fig. 2;

Fig. 5 is a side elevation partly in section of an apparatus. for decomposing and reducing certain.

of my carbon and oxygen containing compounds;

Fig. 6 is a diagram of an apparatus for decomposing carbonyl compounds.

The quality of a permanent magnet is characterized by its coercive force and its remanence, both factors having the highest possible figure in a good permanent magnet.

In Fig. 1. is shown the usual hysteresis loop of a permanent magnet. The abscissae denote applied electromagnetic force and the ordinates the resulant electromagnetic fiux through the magnet. When a permanent magnet is initially magnetized the field indicated by the abscissae to the right of zero is applied until the applied field equals a value indicated at H1 expressed in oersteds. As the field is applied, the flux increases as indicated by the curve F1 until it attains a value B1, expressed in gausses. If the field is then reduced, the flux decreases as indicated by the portion F2 of the hysteresis loop until the field H is reduced to zero, at which point the flux B falls to a value indicated by Br, which denotes the permanent fiux or remanence of the magnet. If an electromagnetic force in the opposite direction is applied as indicated by the abscissae to the left of zero, the flux decreases as shown by the portion F3 of the hysteresis loop until the flux reaches a zero value. The value of the demagnetizing field required to reduce the flux to zero is indicated as He and is termed the coercive force of the magnet.

The coefiicient of quality of a permanent magnet is given by the value of the product of the flux and the applied demagnetizing field at that point of the portion F3 of the hysteresis loop at which this product is a maximum, and which is known as BH maximum, denoted in Fig. 1 as (BH) max. 7.

The curve K indicates as its abscissae the values of theproducts of B and H for different values of H and has a maximum value K1 which corresponds to the point (BH) max. 7

For general purposes it is usually preferred to have the coefficient of quality or BI-I maximum as large as possible. However, for certain purposes it may be desirable to somewhat increase the coercive force at the expense of the remanence, or vice 'versa. I have found that this may be done. by varying the temperature of reduction of the selected ferormagnetic metal compounds, and also by varying the compacting pressure.

In Fig. 3 I have indicated by the intersecting lines H0 and Br the coercive forces and remanences, respectively; for varying temperatures; of. reduction of a series 013- similarly shaped magnets made by the cold compression at metric tons per square centimeter of an iron powder formed by the decomposition. reduction of. iron formate. It will be noted that as the temperature of reduction is decreased within; thelimits shown in Fig. 3, thecoerciveforce isin: creased, and that as the temperature ofreduction is increased, the remanence is increased.

In Fig. 2 I have shown by thepointsh Z and}:

the respective coefficients of quality 0? three. similarly shaped magnets made by compacting. an iron powder formed bathe decomposition and reduction of iron formate for reduction temperatures of 300, 325 and350C.

In Fig. 4 I have shown. by the reference numerals l, 2 and 3 the curves of demagnetization of said magnets respectively. The three curves I, 2' and 3' correspond to the portion of the hysteresis loop in the upper leit quadrant, as indicated by reference letter F3, in Fig. 1.

As as can be seen by an inspectionof Figs. 2 and 4, the magnet made from the iron formate reduced at 325 C. has the greatestcoefiicientof quality (1x10 a coercive force of 4l5 oersteds and a remanence of 6100 gausses. The magnet made from the iron formatereduced. at300 C. has a lower coeflicient of quality (092x a higher coercive force (520 oersteds) and a lower. remanence (4300 gausses) while the magnet made from the iron formats reduced at 350? C. has a lower coefficient of quality (035x10 a lower coercive force (300oersteds) anda higher remanence (7200 gausses),

I will next describe a preferred embodimentof my invention, together with the preferred apparatus for carrying out my. pro cess, as illustrated in Fig. 5.

The starting material is ironformate, preferably in granular form, indicated by reference numeral 10, which is supplied to the feed hopper M. It is fed-through a. regulating valve It to a decomposer indicated generally by. reference numeral l3. Thedecomposer has a.closed.stationary outer shell l4 lined-withelectrical heaters 15. Within the shell Mis-a-rotating thin-walled, metal tube l6 throughwhich the iron forma-teis fed. The iron formats isheated to a temperw ture of preferably about 300, to 325? Q. At this temperature the ironformate decomposes, Gaseous carbon dioxide and carbon monoxide are driven off and a powder isproduoed winch consists of a mixtureoi iron oxides and; asubstam tial amount, general1y-about 29%, of. pure iron, together with a small but substantial; amount, generally about 1%, of carbon. This powder is produced in ahi hly umtahla pndit cn; alldl l so pyrophoric that it must be proteeted againstoxygen.

The carbon dioxide and carbon monoxide-gases are drawn ofi by an exhaust fan, l1, Thepowden falls into the feed hopper ill, from which itis fed through the regulating. valve l9;to the reducer indicated generally by reference numeral, 20.

The decomposing retort 13, thefeedhopper. l8. and connecting piping areenclosed byinsulating jackets 2| so that thepowder is delivered to the reducer in heated condition.

The reducer 20 consists of. a metal hopper-like retort 22 surroundedby an insulating jacket 2-3. The retort is supplied with a continuous circulation of dry purified hydrogen through an intake 24 at the bottom of the retort. Thestream of hydrogen is supplied under 'suflicient pressure and I in: sufiicient amount to, lift, and; agitate the powder-in the retort to insure thorough and intimate contact therewith by the hydrogen gas. lhe hydrogen is supplied in heated condition so as to; maintain. the; powder in the retort at a temperature of about 325 C. An excess of hydrogenis used not onlyto reduce the powder but also to; carry away the products of reduction, such as water vapor as well as the gaseous oxides of carbon which may have been entrained from the decomposing. reaction.

The resultant powder which is drawn off from time totime through the valve 25 into the closed container 26 consists substantially of pure iron, together with about 1% of carbon partially combined with the iron, and-traces-of oxygen, probably as unreduced iron oxide; pyrophoric and must be protected from the air by known methods, such as mixing with dehydratedacetone or gasoline. For this purposea bath 21 of dehydrated acetone or gasoline is maintained in the container 26 into which the powder is discharged.

The heated dry purified hydrogen may be supplied to the reducer 20-by anysuitable and conventional apparatus. In the drawings I have shown such apparatus. Hydrogen is supplied bya suitable hydrogen generator (not shown) to a storage tank 28' illustrated as of the conventional gasometer type. Hydrogen is withdrawn from the tank through the outlet pipe 29- and a compressor 30, from which it flows through pipe 3|- to a purifier 32 which consists of silica gel'and phosphorous pentoxide. The dried purifiedhydrogen next passes througha heater 33 in which,

its temperature is raised to a temperature usually about 400 C., sufi icient. to maintain the powder in the reducer at about 325 C. The heated hydrogen is then injected through the nozzle 2 4 into the bottom of the reducer retort 22;. Thehydrogen issupplied from thenozzle 24 under sufficient pressure, usually about 200 gr. per square centimeter, so asto cause a thoroughintermixing of the hydrogen andtheiron powder. The hydrogen is discharged from thetop of the retort 22 through a pipe 34 and returned to thestorage tank 28 for re-use.

The iron powder which, is, collected in thecontainer 26. is next shapedinto magnetform by compacting at room temperature under a, pres-. su e-ct p ef bm of. heor e o fi metr c. tons per square centimeter, resulting in a density of about 4.5 grams per cubic centimeter. The dehydrated acetone or gasoline accompanying the powder is expelled in the course of the come pacting operation.

The powder may be compacted withor. without. binders such as Bakelite, tars and oils. Such binders serve to seal the magnets against the access of oxygen sincethey coat the iron particles and seal the pores of the compacted. powder. Where the magnets are compacted. without. a binder, the shaped magnet should be impregnated, at least the surface, with some material which will close thepores'and Scalthe; magnet, against the access of oxygen. Such sealingmaterials may be. pitches, resins, greases, plastics or. the like.

The magnet is preferably magnetized after: compacting the powder into magnet shape, although magnetization may be applied during the compacting operation.

A permanent magnet as made by the procedure specificallydescribed has adensity of about 4.5, a coercive force of. about4251oersteds, ends.

The powder remanence of about 6000 gausses, and a coefiicient of quality of about 1,000,000.

The temperature of reduction of 325 C. of the iron formate and the compacting of the reduced iron powder to a density of about 4.5 produces a balance of coercive force and remanence which results in about the maximum coeflicient of quality. There are certain variables which may be controlled to vary the coercive force and the remanence. In general, if these variables are controlled to give a higher coercive force it is done at the expense of a reduction in the remanence, and vice versa.

For example, an increase in the compacting pressure results in a higher remanence with a reduction of the coercive force, and vice versa. While a compacting pressure of about metric tons per square centimeter and adensity of about 4.5 produces about the maximum coefficient of quality, the density may vary from 4.2 to 4.8 and the compacting pressure from about 5 to 8 metric tons per square centimeter. The compacting pressure required to attain a certain density will vary somewhat depending upon the size and shape of the magnet.

Another example of the control of the coercive force and remanence alluded to above is by control of the temperature of reduction of the iron powder, as illustrated in Figs. 2, 3 and 4. The temperature of reduction recited in the specific example is 325 C. The best range of temperature of reduction of iron formate is from about 300 to 350 0., between which limits excellent coefficients of quality are attained. This range may be extended from about 250 to 400 (3., within which range magnets having good to fair coeificients of quality are obtained.

The temperature of reduction as distinguished from the temperature of decomposition appears to be the governing factor in the. control of co.- ercive force and remanence. However, the temperature of decomposition should not be much above that of reduction and for eificient decomposition should not be much below that of reduction. The temperature of decomposition is preferably approximately equal to that of reduction.

While I prefer to carry out decomposition and reduction as separate steps, since this is more economical of hydrogen, decomposition and reduction may be carried out in one operation. In such operation preliminary decompositionis omitted and the ironformate or other compound to be decomposed and reduced. is subjected to the action of a counter current stream of hydrogen in an externally heated tubular retort of the type shown at [6 in Fig. 5, in which decomposition and reduction proceed simultaneously. Much more hydrogen is required since the hydrogen must sweep away not only the gaseous products of reduction but also the gaseous products of decomposition.

While I prefer to use dry purified hydrogen as the reducing gas, other reducing gases may be employed, such as carbon monoxide, or cracked ammonia. Such gases should be freed from water vapor and gaseous oxides.

While I prefer to employ formates because of the low temperatures required for their decomposition and reduction, other carbon and oxygen containing, compounds of ferromagnetic metal may be employed, which require a relatively low temperature for decomposition and reduction, below 500 C and preferably below 400 or 450 0. Among such compounds. are

'. in Fig. 5. ,Iron oxalate requires a somewhat higher temperature for reduction than iron formate. As shown by the curve marked O-xalate? in Fig. 2, reduction of iron oxalate at about380 C. produces compacted magnets 01 the maximum coeflicient of quality. As in the case ofthe formate, a lowering in the temperature of reduction results in an increase in the coercive force and a decrease in the remanence, and an increase in the temperature of reduction results in a decrease of the coercive force and an increase in the remanence. In each case the coefficient of quality is somewhat reduced. The preferred range of temperature of reduction of iron oxalate is between 350 and 375 C'., but this range may be extended from about 300 to 450 C. and good to fair coefficients of quality attained.

Another compound which may be similarly employed isv the carbonate of ferromagnetic metal. The optimum temperature of reduction isabout 380 C. The preferred range of temperature is between 375 and 400 C. Such rangemay be extended from about 350 to 450 C. and good to fair coeflicients of quality attained.

=Other compounds which may be employed are the carbonyls of ferromagnetic metals. The carbonyls are preferably simultaneously decomposed and reduced in an oil bath heated to a temperature of about 275 C.

I have shown at Fig. 6 a preferred embodiment of an apparatus for the decomposition of such carbonyls.

I have shown at Fig. 6 a diagram of an apparatus which may be used for such a decomposition and reduction. A bath of liquid iron carbonyl, indicated by the reference numeral 40, heated to a temperature of about C. in a container 4! surrounded by heating means comprising an insulating casing 42 provided with electric heaters 43, while a current of dried and purified hydrogen is introduced in said bath by means of a tube 44. The iron carbonyl is thus caused to evaporate and the hydrogen containing. the carbonyl vapor is driven off through a tube 45 enclosed by an asbestos insulating jacket 46 and then passed into a bath of mineral oil 41 contained in a tank 48 surrounded by heating means comprising a refractory casing 49 provided with electric heaters 50 and maintained at a temperature of about 275 C. where decomposition and reduction of the carbonyl take place to yield metallic iron containing, as in the case of the reduced formates, oxalates and carbonates, a small but substantial proportion of carbon, usually about 1/1%, together with traces of oxygen. The mixture of mineral oil and iron powder is evacuated through a pipe 5 I, whereafter the mineral oil is washed from the iron powder by a suitable solvent such as dehydrated acetone or gasoline and the powder compressed into magnet form. Other inert liquids which can be heated to the required temperature may be employed in place of the mineral oil. The optimum temperatureof reduction is about 300 C. The preferred range is from about 250 to about 350 C., which range may be extended from about 200 to about 500 C. and good to fair coeflicients of quality attained.

,The reduced powders may contain not only iron but other ferromagnetic metals such as nickel.

and: cobalt or their alloys. Other metalsknown as. entering. into the composition. of ferromagnetic alloys may be addedin powdered form, preferably. in the proportions in which they have been used as alloying metals, such as chromium, manganese, aluminum, bismuth, titanium, molybdenum, tungsten, copperxand the like. Such metal. powders whenused shouldihave their elementary grains or crystals or exceedingly. fine dimensions, of the order of magnitude of those of colloidal particles, for instance of the order of .01 to 0.1 micron.

When it is desired to use powders containing two or more metals it is preferable to decompose and reduce compounds containing such metals to obtain the said'powder. For example, a solution may be prepared containing iron formate. and cobalt formate and crystallized by cooling to produce syncr-ystals ofboth salts, which. are

decomposed and reduced to produce an ironcobalt powder. When in the claims I use the expression ferromagnetic metal I mean to include not only the metals but mixtures thereof.

I have found-that the lower the. temperatures of reduction, the higher the coercive force of magnets made from the reduced powder. It is quite remarkable that particles of substantially pure iron havea coercive-force comparable tothat attained by the quenching of hard alloys usually employed-for the manufacture of permanent ma nets. I believe that this is due to the structure of theparticles of iron or other ferromagnetic metal which I employ. I believethat the metal.

particles as reduced are probably porous, having a sponge-like structure with many holes in it, and that between these holes are very small elementary grains or crystals which constitute the sponge walls. Measurements of the enlargement of the X-ray diffraction lines indicates-that the elementary grains or crystals have dimensions of the order of magnitude of colloidal particles of about .01 to 0.1 micron. I further believe that because of my processcarbon and oxygenare present at the surfaces of suchelementary grains or crystals and inhibit crystal growth and the colloidal dimensions of the elementary grains.

or crystals are largely retained in completed magnet, imparting to it its high coercive force.

Only those compounds of ferromagnetic metal with carbon and oxygen which are reducibleat relatively low temperatures are suitable as startingmaterials in my process. Such compound should be reducible at temperatures below500 0., preferably below 400 or 450 C. Because of the low temperature of decomposition andreductionof the formates, the preferred compounds in carrying out my process, the reduction is preferably carried out at about the lowest temperature consistent with speed of commercial production. The temperature of reduction should not exceed 500 C. and isipreferably below 450 or even 400 C., but Ibelieve that by thus maintaining a low temperature of reduction, small but efiectiveamounts of carbon and oxygen are retained in the reduced powder and inhibit crystal growth.

The reduction is preferably carried to the point where but traces of oxygen or oxides are present in the reduced powder. However, the reduction may lee-incomplete and a minor but substantial amount, say a few per cent of the oxide or oxides of the ferromagnetic metal may be present along with the reduced metal.

The temperature of decomposition, when de-- composition and-reduction are carried out as separate steps, shouldibe held, to. about thesame limits as that oireduction.

The compacting. is preferably carried out cold. It. should not exceedv the temperature of reduction because of the tendency to crystal growth at thehigher temperatures.

Certain examples will now be given to illustratevarious manners, of carrying out the invention.

Example. 1

A molding powder was obtained by the decomposition of iron formate at 320 with simultaneous. reduction with hydrogen for one hour. The quantity of hydrogen usedwas .1 gram per minute per gram of formate. The reduced powder was. bedded in acetone as soon as it was formed, then. taken. out and compacted, in the cold. A magnet compacted to a density of 3 rams per cubic centimeter had a coercive force of 530 oersteds and a remanence of 3000 gausses. Another magnet compacted from the same powder to, a density of 6.5 grams per cubic centimeter had acoercive force of 410 oersteds and a remanence of 5700 gausses.

Example 2 A molding powder was obtained by the decomposition and reduction of cobalt formate under conditions similar to those described for iron formate in Exampleql. Magnets produced from this; powder compacted to a density of 6 grams per cubic centimeterhad a coercive force of 350 oersteds. and. a. remanence of 5700 gausses.

Example 3 A solution containing iron formate and cobalt formate. was; made up proportioned to contain 2'7 partsofcobaltto 73 parts of. iron. This was caused to crystallize by cooling to produce syncrystals containing both salts. Decomposition and simultaneous reduction was effected as in Examplel.

The conditions were varied as follows:

(a): The reduction temperature was 350" C. and the. reduction lasted 45 minutes. The hydrogen used was about .02 gram per minute and pergram of. salt being treated; Two magnets were made from the reduced iron-cobalt powder. The first had a density of 2 grams per cubic centimeter, acoercive force of1400 oersteds and a remanence of 1225gausses. The second had a density of 4 grams per Cubic centimeter, a coerciveforce of 1240. oersteds and a remanence of 2500 gausses.

(b) The hydrogen delivery was increased to about .1: gram. per minute per gram of salt. Two magnets were made. Thefirst hada density of Zagrams per cubic centimeter, a coercive force of 1300"oersteds and a remanence of 1900 gausses. The second had a density of 4 grams per cubic centimeter, a coercive force of 920' oersteds and a. remanence of 3600 gausses.

(c) The reduction temperature was increased to 380 C; and. the duration of reduction reduced to 30 minutes. The hydrogen delivery was about .06 gram per minute per gram of salt. These magnets were compacted from the powder. The first hada densityof 2' grams per cubic centimeter, .a coercive force of 1200 oersteds and a remanence of 2900 gausses. The second had adensity of 4, a. coercive force of 690 and a remanence of. 6200. Thethird had a density of 3,1026 coercive force of 560 and a remanence of A comparison of the results of (a) and (b) Shows that the increase of hydrogen delivery, other conditions remaining the same, caused an increase of remanence and a slight diminution of coercive force.

A comparison of (b) and shows that an increase of the reduction temperature accompanied by a diminution of the duration of reduction caused an increase of the remanence and a small diminution of the coercive force.

In (0) is shown the influence of the degree of compression.

Example 4 A solution containing iron formate and cobalt formate was prepared, with iron and cobalt content of 95 and 5 parts, respectively. The decomposition and reduction was carried out as described above. was 320 C., the duration of reduction was one hour and the delivered quantity of hydrogen about .12 gram per minute per gram of salt. A magnet compacted to a density of 5.5 grams per cubic centimeter had a coercive force of 510 oersteds and a remanence of 715-0 gausses.

EmmpZe 5 An iron powder was prepared by bubbling a mixture of iron carbonyl vapor and hydrogen through an oil bath at 300 C. A magnet compacted from this powder with a density of 5 grams per cubic centimter had a coercive force of 450 oersteds and a remanence of 4200 gausses.

While I have specifically described certain embodiments of the invention, the invention is not limited thereto but may be otherwise embodied and practiced within the scope of the following claims.

What I claim is:

1. The process of making permanent magnets which comprises decomposing and reducing at a temperature below 500 C. a carbon and oxygen containing compound of ferromagnetic metal which is reducible at such temperature to produce a ferromagentic powder containing carbon of the order of 0.2 to 2.5% and traces of oxygen, shaping magnets from the powder by compacting it at a temperature below 500 C. and at which objectionable crystal growth does not occur, and magnetizing the so shaped agglomerate.

2. The process of making permanent magnets which comprises decomposing and reducing at a temperature below 450 C. a carbon and oxygen containing compound of ferromagnetic metal which is reducible at such temperature so as to produce a ferromagnetic metal powder containing carbon of the order of 0.2 to 2.5% and traces of oxygen in which the elementary crystals are of colloidal dimensions as indicated by X-rays, shaping magnets from the powder by compacting it at a temperature below 450 C. and at which objectionable crystal growth does not occur and magnetizing the so shaped aglomerate.

3. The process of making permanent magnets which comprises decomposing at a temperature below 500 C. and subsequently reducing by means of a reducing gas at a temperature 'below 500 C. a carbon and oxygen containing compound of ferromagnetic metal which is reducible at such temperature so as to produce a ferromagnetic metal powder containing carbon of the order of 0.2 to 2.5% and traces of oxygen, shaping magnets from the powder by compacting it at a temperature not higher than that at which The reduction temperature 10 it was reduced, and magnetizing the so-shaped agglomerate.

4. The process of making permanent magnets which comprises decomposing at a temperature below 450 C. and subsequently reducing by means of dry purified hydrogen at a temperature below 450 C. a carbon and oxygen containing compound of ferromagnetic metal which is reducible at such temperature so as to produce a ferromagnetic metal powder containing carbon of the order of 0.2 to 2.5% and traces of oxygen, shaping magnets from the powder by compacting it at a temperature not higher than that at which it was reduced, and magnetizing the so shaped agglomerate.

5. The process of making permanent magnets which comprises decomposing iron formate at a temperature between 275 and 400 C. and subsequently reducing it by means of dry purified hydrogen at a temperature between 275 and 400 C. so as to produce an iron powder containing carbon of the order of 0.2 to 2.5% and traces of oxygen, shaping magnets from the powder by com- ,pacting it at a temperature not higher than that at which it was reduced, and magnetizing the so shaped agglomerate.

6. The process of making permanent magnets which comprises decomposing a solid pulverulent carbon and oxygen containing compound of ferromagnetic metal which is reducible at a temperature below 500 C. through heating it in a closed vessel at a temperature of about 300 to 325 C., while permanently extracting the gases resulting from said decomposition, transferring in a second vessel the decomposed powder obtained, passing through said decomposed powder at a temperature of about 325 C. and under a sufiiciently high pressure to insure a thorough mixing with said powder, a current of dried and purified hydrogen in excess to produce a ferromagnetic powder containing carbon of the order of 0.2 to 2.5% and traces of oxygen, transferring said ferromagnetic powder directly into a liquid protecting it from oxidation, shaping magnets from the powder by compacting it at a temperature below 325 C. and at which objectionable crystal growth does not occur, the hydrogen passed through the decomposed powder being recycled after having received the necessary quantity of fresh hydrogen and being purified and dried, and magnetizing the so shaped agglomerate.

LOUIS NE'EL.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,044,763 Ekstrom Nov. 19, 1912 1,174,727 Jones Mar. 7, 1916 1,865,180 Faragher June 28, 1932 1,982,689 Polydoroff Dec. 4, 1934 1,986,197 Harshaw Jan. 1, 1935 2,096,009 Schmid Oct. 19, 1937 2,120,958 Coons June 14, 1938 2,196,824 Dahl et al Apr. 9, 1940 2,239,144 Dean et a1 Apr. 22, 1941 2,315,302 Volterra Mar. 30, 1943 2,427,018 Nesbitt Sept. 9, 1947 FOREIGN PATENTS Number Country Date 423,823 Great Britain Feb. 8, 1935 239,381 Switzerland Feb. 1, 1946 590,392 Great Britain July 16, 1947 610,514 Great Britain Oct. 18, 1948 

1. THE PROCESS OF MAKING PERMANENT MAGNETS WHICH COMPRISES DECOMPOSING AND REDUCING AT A TEMPERATURE BELOW 500* C. A CARBON AND OXYGEN CONTAINING COMPOUND OF FERROMAGNETIC METAL WHICH IS REDUCIBLE AT SUCH TEMPERATURE TO PRODUCE A FERROMAGNETIC POWDER CONTAINING CARBON OF THE ORDER OF 0.2 TO 2.5% AND TRACES OF OXYGEN SHAPING MAGNETS FROM THE POWDER BY COMPACTING IT AT A TEMPERATURE BELOW 500* C. AND AT WHICH OBJECTIONABLE CRYSTAL GROWTH DOES NOT OCCUR, AND MAGNETIZING THE SO SHAPE AGGLOMERATE, 